It's hard to tell if the saddle fits but you might want to go a half size up. I am 5'6" and I use a 17.5...it's all about measuring your femur...Take a measuring tape and measure your femur from where your butt and leg meets to the lower part of the femur and you will find your seat size. Also brands differ in sizes sometimes.
You can be a skinny thing and ride in a 18"..just depends on your leg..
You will get your leg down...i use to have a really bad forward seat...also had a 17" that was a bit too small...got a new saddle and fits well.
The length of an English saddle is set by the length of the horse's spine and rib cage.
The angle of the width of the saddle is determined by the broadness of the horse's back. Saddles come: narrow, ordinary, wide, extra wide.
The horse's wither & spine must be kept free from pressure at all times.
All the rider has to feel is: 'comfortable'.
Some saddles have high cantles which provide a deep seat
Some saddles have cutback pommels to allow for a high wither.
Jumping saddles have deep knee rolls.
Dressage saddles are stright cut
The presence or absence of kneerolls, and the shape of skirt, are determined by the type of saddle and the use predicted for it.
The stirrup leathers allow the ball of the foot to rest on the bar of the stirrup iron - with the heel laying slightly lower than the toe. Buy synthetic leathers rather than pure leather which stretch with use.
The rider's leg should hang down naturally - unless the stirrup leathers are deliberately adjusted short for jumping or cross country riding.
A well fitting saddle, preferably as determined by a saddle fitter, is the secret of a correct riding position. The saddle fits primarily the horse not the rider.
Leather saddles require re-stuffing from time to time in accordance with the shape of the horse back. Some leather saddles have a greater depth of soft stuffing than others.
Look up the Ideal Saddle Co UK website - all the info you need is there.
English saddles made in England are sold as:
General Purpose, Hunting, Show-Jumping, Dressage & Endurance.
Each has a different cut.
Brits expect that when they buy a new horse - it might need its own new saddle.
A saddle pad is not designed to make an ill fitting saddle fit - it is designed to ease and soften the weight of the rider on the horse's back.
The saddle should fit the horse without any padding.
If the horse fattens up thru lack of work or alternatively if it loses weight and muscles up - then the saddle will need re-stuffing.
Remember the 'foot'print of an English saddle is small compared with a Western saddle. The pressure per square inch is greater so the English saddle must match exactly the shape of the horse's back. It must sit flat and horizontal with the rider's weight equally dispersed front to rear, side to side.
It must not rock front to rear.
I believe that the saddle fit my long-backed horse well, it's solid on her and fits just as good as my western saddle does, even though she doesn't have much for withers... The saddle sits off of her wither and for what I can see, she doesn't fuss about any issaues with the fit.